Almost all business and home computers use a HDD storage system for mass storage requirements. A HDD stores data by individually modifying the magnetic orientation of small regions of a disk surface. A HDD typically includes one or more rotating disks. A head assembly associated with each surface of the disks typically includes separate read and write heads for reading data from the disk and writing data to the disk. The write head is essentially a small coil of wire which stores data by magnetizing small regions along a disk's tracks. During a write operation a current driven through the write head in a first direction magnetizes a small region of the disk under the head at a first orientation and a current driven through the write head in the opposite direction magnetizes a small region of the disk under the head at a second orientation. During a read operation, the read head distinguishes the magnetic orientation of each bit location to derive logical “1s” and “0s”.
The circuit which drives the write head is commonly referred to as a “write driver” which is generally a part of the read/write preamplifier circuit. The write driver controls the direction and timing of the flow of current flowing through the inductive write head, responsive to information received from circuitry commonly referred to as channel circuitry. The channel circuitry generally receives data from the hard drive controller of a computer.
Write drivers drive the write head differentially to achieve the maximum voltage possible across the write head for both positive and negative transitions. The requirement of driving the write head differentially means that both sides of the write driver must have bidirectional current drive capability.
As the frequency of the write driver current pulses is increased, so does the density of the stored data. Since it is desirable to increase the density of stored data, it is likewise desirable to increase the frequency of the current write pulses. The time required for the current to change from its present value to its next value (rise and fall times) ultimately limits the frequency of the current write pulses, therefore it is desirable to reduce the rise and fall times. Accordingly, if the rise/fall speed of the current applied to the write head increases, the writing speed increases.
Pulse switching write drivers generally operate in an open loop configuration for highest speed. For such open loop drivers it is very difficult to prevent saturation and cutoff of the H-bridge transistors as switching occurs. Low power coupled with high switching speed often results in dead bands and saturated H-bridge transistors which become sluggish as they recover from the present state to become able to deliver power to the inductive load presented by the write head responsive to the subsequent switching pulse. New write driver designs are needed that provide higher switching speed, while at the same time providing low power operation.